A stripline filter having filter characteristics suitable for a communication system that uses a wide band in a high frequency band, is known (see Patent Document 1).
FIG. 1 shows a configuration of an existing stripline filter. The stripline filter 101 is a filter that uses three resonators. The three resonators are constituted of lines 102, 103A, and 103B, respectively, provided on the same principal surface of a dielectric substrate. The line 102 is bent in U shape, and its ends are open. The lines 103A and 103B each has an I shape in which one end is connected to a ground electrode 105, and their ends are open. These resonators are interdigitally coupled to each other, and input/output transmission lines 104A and 104B are connected to the lines 103A and 103B, respectively. In this configuration, the resonators are strongly coupled to each other by being interdigitally coupled to each other, thereby achieving expansion of the band of the filter characteristics.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2001-358501
Due to development of UWB (ultra wide band) communication and the like, there is a demand for further expansion of the bands of stripline filters. However, in existing stripline filters, due to constraints on element size and the like, there are limitations on the line lengths of striplines and the degree of coupling between resonators, and there is also a limitation on expansion of bands.
Moreover, it is difficult to optionally set an attenuation pole and the like while achieving wide-band frequency characteristics. Particularly, it is difficult to provide an attenuation pole on a high frequency side of the band and to finely set its pole frequency.